Answer
About $4$ inches.
Work Step by Step
A square with area 15 square inches will have side lengths that are the square root of the area.
This is because, for a square, Area = $s^2$, so $s=\sqrt A$
With an area of 15, the side will be $s=\sqrt {15}$
Since the problem asks us to estimate, we can say this is about $4$.
